1. Field of the Invention
The present invention relates to a method of making a printed board having a base having hollow portions closed by resin fillings. Further, the present invention relates to a method of making a printed board having a base having through via conductors having through holes and blind via conductors having depressions, which through holes and depressions are closed by resin fillings. Further, the present invention relates to a method of making a printed board which is capable of preventing, after curing of resin paste filled in the hollow portions as through holes and depressions, the surfaces of cured resin fillings from receding or sinking and becoming lower in level than the surface of the base.
2. Description of the Related Art
Heretofore, in order to connect between conductive layers formed on upper and lower surfaces of a printed board or conductive layers formed inside the printed board, it has generally been practiced to provide the printed board with through via conductors and blind via conductors for connecting between the conductive layers. In manufacture of such a printed board, the through holes of the through via conductors and the depressions of the blind via conductors are closed by resin fillings so that an insulating resin layer placed on a layer having such via conductors can be flat or planar.
In this connection, a prior art process of closing the through holes of the through via conductors and the depressions of the blind via conductors by resin fillings is by way of example shown in FIGS. 9A-9B and 10A-10B. As shown in FIG. 9A, it is prepared a base 51 which is provided with through via conductors 53 extending completely between an upper surface 51A and a lower surface (not shown) of the base 51 and having through holes 52 therewithin, blind via conductors 55 disposed on the upper surface 51A and having depressions 54, and internal conductors 56 disposed inside the base 51. The surface 51A of the base 51 prior to being formed with a conductive layer is entirely covered by and therefore defined by a metallic layer 57. On the surface 51A is placed a mask MS having through holes HL corresponding in position and shape to the through holes 52 and the depressions 54, and then printing is performed for filling, by way of the mask MS, the through holes 52 and the depressions 54 with resin paste 61.
Thereafter, the base 51 is heated to cure the resin paste fillings 61 for thereby forming cured resin fillings 62 in the through holes 52 and the depressions 54.
As shown in FIG. 9B, after curing, the surfaces of the resin fillings 62 in the through holes 52 and the depressions 54 are caused to recede or sink and become lower in level than the surface 51A of the base 51. Such receding is caused because a portion of each resin paste filling 61, protruding from the surface 51A of the base 51, is heated to melt and spread over the surface 51A of the base 51, thus causing the resin paste fillings 61 within the through holes 52 and the depressions 54 to decrease and lack in quantity, that is, a so-called bleed-out is caused. If such resin paste fillings 61 are cured, the surfaces of the cured resin fillings 62 are caused to recede to form receding spaces thereon. Further, it is considered that in case contraction is caused by curing, such receding or sinking is accelerated by the contraction.
Due to this, in order to close the spaces receding beyond the surface 51A of the base 51, the following process is necessitated. That is, resin paste 63 is additionally applied to the entire surface 51A of the base 51 as shown in FIG. 10A. Then, the base 51 is heated again to cure the additionally applied resin paste 63 to form cured resin 64. Thereafter, the layer of cured resin 62 and 64 on the surface 51A of the base 51 is ground in such a manner that the surface 51A of the base 51 is flush with the surfaces of the fillings made up of the cured resin 62 and 64.
That is, in order to close the through holes 52 and the depressions 54 of the base 51, the above described process requires to carry out application of resin paste 61 and 63 by printing twice and accordingly to carry out a curing step twice, thus increasing the number of manufacturing steps and making higher the cost of the printed board.